Typical thermoplastic fluoropolymer powder coatings are characterized by degrees of mechanical performance and surface roughness or "orange peel" associated with the finish. "Orange peel" may be reduced through various approaches. While the resin melt viscosity may be reduced in order to combat orange peel, melt viscosity reduction generally has an adverse effect on the mechanical properties of the coating, particularly impact resistance, flexibility, and crack resistance.
Powder coatings of PVDF and copolymers thereof containing at least 90 wt. % VDF are disclosed in U.S. Pat. No. 4,770,939. The PVDF-based resin disclosed in the aforesaid patent for powder coatings has a melt viscosity according to ASTM D3835 of 6 to 7 kilopoise at 230.degree. C. A minor amount of a low molecular weight acrylic polymer is required as a flow improving agent.
European Patent Application 284,996 discloses a powder coating composition of a polymeric binder and greater than 40 wt. % of a coalescing additive component containing at least one coalescing agent which is solid at ambient temperature to promote flow and fusion of the binder into a substantially coalesced film with the application of heat. The coalescing additive is substantially volatilized from the film during coalescence. The release of such latent solvents is undesirable.
What is needed is a fluoropolymer powder coating characterized by a very low viscosity such that the inclusion of flow-improving agents is unnecessary. There is a need for a coating composition which is smooth and yet retains desirable mechanical properties of impact and crack resistance.